Determination of the chemical mechanism of chromate conversion coating on magnesium alloys EV31A

Highlights

• We discuss the chemical mechanism of chromium conversion coatings (CCC).

• We identify the functions of each chemical species composing the coating.

• We identify the role and the influence of each step of the process of CCC.

• We evaluate the corrosion properties of each compound of the coating formed during the deposition process.

Abstract

Magnesium and its alloys present several advantages such as a high strength/weight ratio and a low density. These properties allow them to be used for many aeronautical applications but they are very sensitive to corrosion. To solve this problem, conversion coatings are deposited on the surface before a protective top coat application. Several kinds of coatings exist but the best protective is chromium conversion coating (CCC). This process is now limited by several environmental laws due to the high toxicity of hexavalent chromium. However, in order to reduce hazardous impact onto the environment and to find alternative coatings, the chemical mechanisms of CCC deposition and protection on magnesium alloy are detailed for the first time in this work. The studied process includes 4 pre-treatments steps and a conversion immersion bath. The pre-treatment steps clean and prepare the surface for improving the coating deposition. The coating properties and its composition were characterized by voltammetry and XPS technics. A final layer of chromium(III) oxide and magnesium hydroxide composed the coating giving it its protective properties. Trapped orthorhombic potassium chromate has also been identified and gives to the coating its self healing property.

Introduction

Magnesium alloys possess the lowest density of all metallic constructional materials and have good mechanical properties [1], [2]. They are suitable for the partial replacement of aluminum alloys for motorsport and aerospace applications. This could imply non-negligible weight and fuel savings in the aeronautical sector. However, magnesium alloys demonstrate a high chemical reactivity and limited corrosion resistance [3]. One of the best actual candidates is the EV31A magnesium alloy which gets an excellent castability and shows interesting corrosion resistance (corrosion rate = 0.13–0.37 mg/cm²/day) [4], [5]. This alloy is composed of a main structure made of α grains of magnesium and of an eutectic phase α-Mg + Mg₃(Nd,Gd) on the grain boundaries [6], [7]. The addition of zirconium (Zr) (saturated), gadolinium (Gd) (1.0–1.7%) and neodymium (Nd) (2.6–3.1%) improves the creep properties, thermal stability of structure and mechanical properties at room and elevated temperatures in comparison to other magnesium alloys. In this alloy, zirconium does not form any phase with magnesium or with the other alloying elements but it contributes to a fine grain structure and improves the mechanical properties, castability and corrosion performance. An addition of neodymium to Mg–Gd based alloys reduces the solid solubility of gadolinium. Neodymium has a positive effect on tensile strength at high temperatures, reduces porosity and susceptibility to cracking during welding. Zinc (0.2–0.5%) improves strength without reducing ductility [6]. EV31A has achieved Aerospace Material Specification AMS 4429, MMPDS (MIL-HNDBK-V). It is the first magnesium alloy to enter in the Metallic Materials Properties Development and Standardization Handbook MMPDS entry (MIL-HNDBK-V) EV31A [4].

In spite of these promising specificities a protection against corrosion is still needed. In this way, chromium VI conversion coatings (CCCs) are generally used as they offer an efficient protective solution. Unfortunately, the use of Cr(VI) formulations will be forbidden by 2017 due to the application of the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) legislation [8]. The legislation established last June 2007 by the European Union limits the use of hazardous chemicals and their incidence on human health and environment [9]. This change of legislation partially explains the emergence in the last decade of other coatings developed with phosphate, zirconium, molybdenum and cerium salts [10], even if their anticorrosive performance remains inferior to the chromate conversion coatings (CCC) [11]. New effective coatings need to be adapted to mimic the chemical mechanistic of chromium species in the alloy surface protection. No precise work has been published yet in that way and the chemical form of the hexavalent chromium species, often reported in the case of coating over aluminum alloy, is not precisely identified [8]. Some assumptions suggested that polar oxo-Cr(VI) anions, present inside of the CCC coating, annihilate the adsorption of depassivating anions such as chloride ions [12]. The presence of trapped hexavalent chromium is responsible of the “self-healing” ability of the coating under corrosion which remains a tremendous advantage driven by this species [13]. The determination and the control of the chemical form of hexavalent chromium in the coating is the key to understand the properties of the chromate films and to find out some equivalent coatings in the years to come.

In this work, different chromium-based conversion coatings have been obtained for an EV31A magnesium alloy. The process of chromate coating deposition was explored through two steps: pre-treatment and treatment. To describe the coating composition during its deposition and its chemical properties, the etching solutions (pre-treatment) and the coatings (CCC treatment) were monitored by electrochemical and spectrometric techniques. The chemical composition and the microstructure features of the protective coated layers were examined by scanning electron microscopy (SEM) and X photoelectrons spectroscopy (XPS).